Many processes, for example combustion processes, or industrial production of nitric acid or caprolactam, result in an offgas laden with nitrogen monoxide NO, nitrogen dioxide NO2 (referred to collectively as NOx) and dinitrogen monoxide N2O. While NO and NO2 have long been known to be compounds of ecotoxic relevance (acid rain, smog formation), and global limits have been fixed for the maximum permissible emissions thereof, dinitrogen monoxide too has become the subject of increasing attention in environmental protection in the last few years, since it contributes to a not inconsiderable degree to the degradation of stratospheric ozone and to the greenhouse effect. For reasons of environmental protection, there is therefore an urgent need for technical solutions for elimination of the dinitrogen monoxide emissions together with the NOx emissions.
There are already numerous known options for elimination of N2O on the one hand and NOx on the other hand.
In the case of NOx reduction, selective catalytic reduction (SCR) of NOx by means of ammonia in the presence of vanadium-containing TiO2 catalysts should be emphasized (cf., for instance, G. Ertl, H. Knözinger, J. Weitkamp: Handbook of Heterogeneous Catalysis, vol. 4, pages 1633-1668, VCH Weinheim (1997)). According to the catalyst, this can proceed at temperatures of approx. 150° C. to approx. 450° C. and is operated on the industrial scale preferably between 200° C. and 400° C., especially between 250° C. and 350° C. It is the variant usually used for reducing the NOx level in offgases from industrial processes, and enables NOx degradation of more than 90%.
There are also processes for reduction of NOx based on zeolite catalysts, which proceed using a wide variety of different reducing agents. In addition to Cu-exchanged zeolites (cf., for example, EP-A-914,866), iron-containing zeolites in particular appear to be of interest for practical applications.
For instance, U.S. Pat. No. 5,451,387 describes a process for selective catalytic reduction of NOx with NH3 over iron-exchanged zeolites, which operates preferably at temperatures between 200 and 550° C., especially around 400° C.
EP-A-756,891 describes a process for reduction of NOx by means of NH3 in the presence of honeycomb monoliths composed of iron-containing ZSM-5 zeolites. An advantage of the Fe zeolite catalysts over conventional V2O5—TiO2-based deNOx catalysts is likewise the extended temperature range from 200° C. to 600° C.
However, a disadvantage of Fe zeolite catalysts for NOx reduction is the availability and cost thereof. The latter is much higher compared to widespread and established V2O5—TiO2-based deNOx catalysts.
In contrast to the reduction of the NOx level in offgases, which has been established in industry for many years, there exist only a few industrial processes for N2O elimination, which are usually aimed at thermal or catalytic degradation of the N2O. An overview of the catalysts which have been demonstrated to be suitable in principle for degradation and for reduction of dinitrogen monoxide is given by Kapteijn et al. (Kapteijn F. et al., Appl. Cat. B: Environmental 9 (1996) 25-64). The catalytic decomposition of dinitrogen monoxide to N2 and O2 gives the advantage over catalytic reduction with selected reducing agents, such as NH3 or hydrocarbons, that no costs arise for the consumption of reducing agents. However, effective lowering of the N2O level based on a catalytic decomposition, in contrast to N2O or else NOx reduction, can be achieved effectively only at temperatures greater than 400° C., preferably greater than 450° C.
Again, transition metal-laden zeolite catalysts appear to be particularly suitable for catalytic decomposition of N2O to N2 and O2 (U.S. Pat. No. 5,171,553).
Iron-laden zeolite catalysts are described as especially advantageous (for example in EP-A-955,080 or WO-A-99/34,901). The activity of the Fe zeolite catalysts for N2O decomposition is enhanced considerably by the simultaneous presence of NOx, as detailed scientifically, for example, by Kögel et al. in Catalysis Communications 2 273-276 (2001) or by Perez-Ramirez et al. in Journal of Catalysis 208, 211-223 (2003).
The combined elimination of NOx and N2O based on a catalytic reduction of the NOx with NH3 (in a deNOx stage) and a catalytic decomposition of N2O to N2 and O2 over iron-containing zeolite catalysts (in a deN2O stage) has also been described in the patent literature.
For example, DE 10 001 541 B4 claims a process for eliminating NOx and N2O from the residual gas of nitric acid production, wherein the offgas to be cleaned is passed first through a deNOx stage and then through a deN2O stage with iron-laden zeolite catalysts. In the deNOx stage, the NOx content is reduced to such an extent that an optimal NOx/N2O ratio of 0.001-0.5 is established, which leads to accelerated N2O degradation in the downstream deN2O stage.
The selected sequence of process stages is very advantageous from a process and chemical engineering point of view, since the process is arranged in the residual gas of the nitric acid production, between absorption tower and residual gas turbine in an ascending temperature profile; in other words, the residual gas at first, before entry into the deNOx stage, has a low inlet temperature which is <400° C., preferably <350° C., and so conventional deNOx catalysts based on V2O5—TiO2 can also be used. The deNOx stage, before entry into the deN2O stage, is then followed by a (single) heating of the residual gas to 350-500° C., such that effective catalytic N2O decomposition is possible. The offgas is then supplied to a residual gas turbine in which the heat content of the offgas is recovered with decompression and cooling.
A reverse connection of the two process stages is also possible, i.e. in a sequence in which N2O degradation is first provided and is then followed by NO degradation, as taught in WO-A-01/51181.
For this purpose, the offgas is passed at a homogeneous temperature of <500° C. through two reaction zones which comprise iron-laden zeolite catalysts and may be spatially separate from one another or connected to one another. In this case, the N2O is decomposed in the deN2O stage initially at an unreduced NOx content, i.e. with full exploitation of the cocatalytic NOx effect on the N2O decomposition, and this is followed, after intermediate addition of ammonia, by the catalytic NOx reduction. Since the NO reduction should preferably proceed at the same temperature as the N2O decomposition, Fe zeolite catalysts are likewise used in the deNOx stage, which, in contrast to conventional SCR catalysts, for example V2O5—TiO2-based catalysts, can also be operated at higher temperatures>400° C. Intermediate cooling of the process gas is thus not required.
If it were desired, for example for reasons of cost, to employ less expensive SCR catalysts, such as V2O5—TiO2-based catalysts, in place of the Fe zeolite catalysts, cooling of the residual gas would thus always be required downstream of the deN2O stage for operation of the deNOx stage. This would be highly advantageous especially when, even in the case of use of other deNOx catalysts, for example Fe zeolite-based catalysts, as a result of the specific application, for example in a plant for preparation of nitric acid by what is called the mono-medium pressure process or, for example, in a plant for preparation of caprolactam, a low exit temperature downstream of the denitrification unit is desired or required.
In this case, the person skilled in the art in the field of offgas cleaning is, however, confronted with the following problem, which makes the operation of a conventional deNOx stage at a low temperature level downstream of a deN2O stage comprising Fe zeolite catalysts appear to be technically and economically very disadvantageous.
For instance, Fe zeolite catalysts are known, as shown, for example, by Kögel et al. in Catalysis Communications 2 273-276 (2001) or by Perez-Ramirez et al. in Journal of Catalysis 208, 211-223 (2002), not only to accelerate N2O decomposition but also, in the presence of NOx, also to shift the NO/NO2 ratio or the degree of NOx oxidation in an accelerated manner. The latter is defined as the molar proportion of NO2 in the total molar amount of NOx (=sum of NO and NO2); in other words, the higher the operating temperature of the deN2O stage, the more rapidly and the greater the extent to which the NO/NO2 ratio approaches the thermodynamically defined equilibrium position at the exit from the stage.
While the formation of NO2 is predominant at low temperatures of <400° C., preferential formation of NO takes place at higher temperatures of >400° C. or especially at T>450° C. (on this subject, see FIG. 1, which shows the mole fractions of NO and NO2 in thermodynamic equilibrium at 1 bar abs proceeding from 500 ppm of NO, 500 ppm of NO2, 2% by volume of O2 and remainder N2).
The formation of NO2 resulting from reaction of N2O with NO at relatively low temperatures, according to reaction equation (1) below, becomes increasingly meaningless since NO2 formed, according to reaction equation (2) below, is degraded again very rapidly to NO.N2O+NO→NO2+N2  (1)NO2NO+½O2  (2)
At the exit of the deN2O stage, that degree of NOx oxidation which corresponds to the thermodynamic equilibrium is thus established at high temperatures.
This relationship is well known to those skilled in the art and is described, for example, in the aforementioned articles by Kögel et al. and Perez-Ramirez et al. According to Perez-Ramirez et al., FIG. 5a on page 215, in a water-free test gas with 1.5 mbar of N2O and 0.4 mbar of NOx, in spite of intermediate formation of NO2 according to reaction equation (1), an NO/NO2 ratio which corresponds to the thermodynamic equilibrium position is established at temperatures of >700 K (corresponding to >427° C.), even at a high space velocity of 60 000 h−1. In the aforementioned article by Kögel et al., FIG. 1 shows that, proceeding from a water-containing test gas containing 1000 ppm of N2O and 1000 ppm of NO, the thermodynamic NOx equilibrium is attained from 400° C. at a space velocity of 15 000 h−1.
This means that, at the exit of a deN2O stage, at T>400° C. and pressure 1 bar abs, a degree of oxidation of <30% should be assumed, and at T>450° C. even a degree of oxidation of <20%. However, such a degree of oxidation is generally extremely unfavorable for operation of a deNOx stage.
Thus, a deNOx stage is known to function at best when the ratio of NO/NO2=1/1, i.e. the degree of oxidation is approx. 50%. In this case, the person skilled in the art refers to a “fast SCR” (cf. reaction equation 3 below), which proceeds several times faster than “standard SCR” (cf. reaction equation 4 below) or “NO2 SCR” (cf. reaction equation 5 below).4NH3+2NO+2NO2→4N2+6H2O  (3)4NH3+4NO+O2→4N2+6H2O  (4)4NH3+3NO2→3.5N2+6H2O  (5)
The dependence of the reaction rate on the degree of NOx oxidation is especially important for the operation of a deNOx stage at low temperatures. This is true both in the case of use of conventional SCR catalysts, such as V2O5—TiO2-based deNOx catalysts, as described, for example, by Koebel et al. in Catalysis Today 73, (2002), 239-247 (cf. FIG. 3 therein), and, for example, of deNOx catalysts based on iron zeolite.
It is evident from this that the operation of a deNOx stage at low temperatures<400° C., preferably <350° C. and especially <300° C., downstream of an N2O decomposition based on Fe zeolite catalysts, is exceptionally disadvantageous since the activity of the NOx reduction, i.e. the performance of the deNOx catalyst in the deNOx stage, is greatly impaired by the unfavorable degree of NOx oxidation.
This disadvantage can be counteracted only to a limited degree by an increase in the amount of catalyst in the deNOx stage, since disproportionately large amounts of additional catalyst would be required to achieve high degradation rates of NOx of, for example, >80% or preferably >90%, especially at high NOx inlet concentrations. This would not only put into question the economic viability of the process due to excessive capital and operating costs, but it would also be unjustifiable in many cases for technical reasons, such as space required or permissible pressure drop.
It is thus an object of the present invention to provide a process for removing N2O and NOx from offgases by catalytic decomposition of N2O by means of iron-containing zeolite catalysts and by catalytic reduction of the NOx by means of reducing agents, wherein the deN2O stage should be operated downstream of the deN2O stage at inlet temperatures of T<400° C., preferably <350° C. and especially of T<300° C., and which overcomes the abovementioned disadvantages.
It is a further object of the present invention to provide an apparatus with which the aforementioned process can be operated and which enables a simple and economically favorable establishment of the operating parameters required for the deN2O stage and deNOx stage.